1. Field of the Invention
The present invention relates to a circuit board and a semiconductor module and it particularly relates to a circuit board provided with a metal plate in a core part.
2. Description of the Related Art
In recent years, along with increasing high function and high performance of LSIs (Large Scale Integrated Circuit), their power consumption is on the increase. With electronic devices getting smaller, mounting boards are also required to be smaller and high-density and multilayered. Accordingly, the power consumption per unit volume (heat density) of a circuit board rises, thus requiring measures to address the increased heat radiation. As a result, a metal plate having high heat radiation is used as a circuit board.
FIG. 10 is a cross-sectional view schematically showing a structure of a conventional circuit board. This conventional circuit board (metal core board) includes a metal plate (metal layer) 102 provided as core material, an opening (through-hole) 102a penetrating both sides of the metal plate 102, a wiring pattern 114 provided, by way of an insulating layer 104, on the both sides of the metal plate 102, and a conductor 110 which penetrates the metal plate 102 via the opening 102a and which electrically connects the wiring patterns 114 on the both sides. Here, the insulating layer 104 is formed in a manner that insulating resin sheets are thermally bonded from the both sides thereof, with the metal plate 102 positioned as a center, under vacuum or reduced pressure. In so doing, the insulating layer 104 flows inside the opening 102a from the both sides so as to be filled. The conductor 110 is formed in a manner that a through-hole 104a is provided in the insulating layer 104 within the opening 102a and copper plating is applied on the inner surface of this through-hole. Note that an insulator 112 is further embedded in the through-hole 104a where the conductor 110 is provided.
Further, in recent years, the small size and thinning of the circuit board is demanded ever strongly. In order to achieve this, the opening 102a provided on the metal plate 102 needs to be made smaller, and the insulating layer and the like need to be thinned.
However, when the opening 102a in the metal plate 102 undergoes a process of miniaturization, void is likely to be formed within the opening 102a when the insulating layer 104 is formed by thermocompression boding. For this reason, even if the through-hole 104a is provided and the conductor 110 is formed in a state where the void stays therein, the film thickness of the insulating layer 104 in a position where the void is still contained becomes thinner locally. This causes a problem that a dielectric breakdown strength between the metal plate 102 and the conductor 110 cannot be ensured. Further, when the insulating layer 104 is thinned, the thinned insulating resin sheets are subjected to thermocompression boding and the insulating layer 104 flows inside the opening 102a. This produces a dent in the insulating layer 104 at an upper region of the opening 102a. Accordingly, the thickness of the insulation layer 104 becomes relatively thin in the vicinity of the opening 102a of the metal plate 102. As a result, the dielectric breakdown strength between the metal plate 102 and the conductor 110 (wiring pattern 114) is likely to be insufficient. In particular, since electric field tends to converge to a corner (opening end) of the metal plate 102, the dielectric breakdown strength becomes more problematic.